Catheter head

ABSTRACT

The present invention relates to a catheter head comprising: means ( 104, 108; 306, 304; 320; 322; 326; 338 ) for directing of radiation to a blood detection volume ( 220; 310 ), means ( 104, 108; 306, 304; 320; 322; 326; 332, 334, 330; 338 ) for receiving of return radiation from the blood detection volume, means ( 104; 306; 330 ) for transmitting of the return radiation to means ( 122 ) for analysis of the return radiation for determination of at least one property of the blood.

FIELD OF THE INVENTION

The present invention relates to the field of catheters and imagingsystems for catheterisation.

BACKGROUND AND PRIOR ART

Catheterisation provides effective and quality service in significantlyreducing patient discomfort, hospital stay, and medical cost. It oftenrequires the ability to enter the vascular system through very smallincisions and to manoeuvre therapeutic or diagnostic devices to thetarget region in a human body. With the smallest possible circularcross-sections, catheters are the most important device widely used ininterventing procedures. More than any other type of interventingdevice, catheters are extremely diverse in shape and specific features.Each catheter is designed for its own purpose and is distinct fromothers with its own characteristics and configuration.

The term catheter as used herein refers to any type of invasive surgicaltool, used for insertion into a human or animal body for the purpose ofproviding remote access to a part of the body for performing some typeof investigative and/or medical procedure.

U.S. Pat. No. 6,208,887B1 shows a catheter-delivered low resolutionRaman scattering analysing system for detecting lesions of a subject.The system uses a multi-mode laser attached to a catheter in makingin-vivo Raman spectroscopic measurements of the lesion. The systemincludes a light collector and a light dispersion element as well as adetector to measure spectral patterns that indicate the presence of alesion. In addition the components of the lesion can also be identifiedbased on the unique Raman spectrum associated with each component of thelesion.

Further, various catheter tracking techniques for remotely locating andtracking a catheter inside a human or animal body are known from theprior art. Currently, X-ray fluoroscopic imaging is the standardcatheter tracking technique. For example the Philips Cath-Lab systemsprovide X-ray imaging during catheterisation for monitoring of theoperation.(http://www.medical.philips.com/main/products/cardiovascular/)

For example, in catheter based surgery a long and narrow plastic tube isinserted into the artery in the groin or arm. The physician then leadsthe catheter through the main artery to the heart During heartcatherisation, the following diagnostic measurements can be made:

-   -   A small amount of contrast dye can be injected via the catheter.        This contrast allows the blood vessels and heart chambers or        valves, to be viewed using X-rays.    -   The pressure inside the heart chambers can be measured.    -   The concentration of oxygen and carbon dioxide in the blood can        be measured locally.    -   The electrical signals inside the heart can be measured, or the        response to applied electrical signals can be determined.

Catheter based treatments include:

-   -   PTCA (percutane transluminale coronaire angioplasty) to widen        the blood vessel locally.    -   Placement of an endovascular prostheses in the blood vessel.

Typically various medical parameters are measured and monitored duringcatheterisation, such as heart frequency, blood pressure and others.This medical information is essential for permanently monitoring thestate of the patients body.

There is therefore a need for a catheter head enabling an improvedmonitoring of the state of a patient's body during catheterisation.

SUMMARY OF THE INVENTION

The present invention provides for a catheter head which enables in vivodetermination of at least one blood property by directing of radiationto a blood detection volume and analysing of return radiation which isreturned from a blood detection volume.

For example the catheter head has an optical wave guide for guidingradiation, such as laser radiation or infrared radiation, to a blooddetection volume which is located in front of the catheter head;alternatively the blood detection volume can be located within an inletor cavity inside the catheter head or at another suitable location.Radiation which is returned from the blood detection volume in responseis captured by the catheter head and transmitted to an analyser. On thebasis of the analysis of the return radiation at least one property ofthe blood is determined.

In accordance with a preferred embodiment of the invention Ramanspectroscopy is utilized. Laser radiation is directed into a blooddetection volume which is located in front of or inside the catheterhead. The resulting Raman scattered radiation is transmitted to aspectroscope for analysis of the Raman spectrum in order to determineone or more properties of the blood.

Raman spectroscopy is based on inelastic scattering of light onmolecules. In this scattering process, energy is transferred between thephoton and the molecule, resulting in a wavelength shift of the light.The energy corresponding to the wavelength shift is equal to the energydifference of vibrational states of the molecule.

By detecting the Raman signal in a sufficiently large wavelength region,the energy of a large number of molecular states can be calculated.Because this combination of energies is specific for each molecule, theRaman spectrum can be considered as a fingerprint of a molecularspecies. Blood analytes—for example glucose or lactate—can be detectedusing Raman spectroscopy. These analytes provide general and specificinformation about the patient's health.

The integration of a Raman probe into a catheter in accordance with thepresent invention has the advantage, that blood analysis can bepermanently performed during catheterisation for improved monitoring ofthe medical state of the patient during the operation. For example, thisdiagnostic tool can be used during catheterisation for the followingpurposes:

-   -   monitoring of the catheterisation procedure, for example by        continuous monitoring of the patient's health by measurement of        oxygenation or lactate;    -   measurement of local blood composition.

This compares to prior art blood analysis, where blood is drawn from thearm using a needle and the blood sample is analysed in a chemicallaboratory. This analysis and the transport take a considerable amountof time, varying between two days and typically 20 minutes in emergencysituations. In contrast the present invention enables to continuouslymonitor the properties of the blood which provides the physician with upto date information on the medical state of the patient.

In accordance with a preferred embodiment of the invention, confocalRaman spectroscopy using optical wave guides is used. Light from theRaman excitation laser is coupled into an optical fibre and this fibreis incorporated into the catheter. In the catheter head, light from thefibre is collected by a lens and is focused into the detection area.Raman scattered light is collected by the same objective and coupledback into the optical fibre. The endpoint of the fibre serves as apinhole to ensure confocal detection.

In accordance with a further preferred embodiment two separate opticalfibers are used: One optical fibre for transmitting the incident lightto the blood detection volume and one for the return radiation.

In accordance with a further preferred embodiment of the invention, alens is used having a high numerical aperture (NA) in order to collectas much Raman scattered radiation as possible. The collected Raman lighttravels back through the optical fibre and is detected by a spectrumanalyser to yield quantitative concentration measurements of thedetected analyte(s).

In accordance with a further preferred embodiment of the invention, thenumber of red and/or white blood cells in the detection volume isreduced in order to reduce absorption and scattering of light. Forexample a mesh with a shutter mechanism can be used for this purpose.

In accordance with a further group of preferred embodiments of theinvention, optical elements are used to enhance the collectionefficiency of the Raman scattered radiation. This can be done by meansof spherical or ellipsoidal mirrors.

In accordance with a further preferred group of embodiments the blooddetection volume is located in a cavity of the catheter head throughwhich blood flows. In order to enhance the flow of blood through thedetection volume the blood channel through the catheter head can bedisposed such to make usage of the Pitot tube effect.

As an alternative to the Raman effect other spectroscopic techniques canbe used. For example this can be done by means of infrared light whichis directed to the blood detection volume. In this instance the returnradiation is analysed by means of infrared absorption spectroscopy whichdetects changes in the infrared light intensity.

In accordance with a further preferred embodiment of the inventionfluorescence spectroscopy is used. In this instance a laser beam oranother kind of radiation is directed to the blood detection volume inorder to excite molecules to emit induced fluorescence. The detectedfluorescence forms the basis for the determination of the at least oneproperty of the blood.

In accordance with a further preferred embodiment of the inventionelastic scattering spectroscopy is used. In this case the variations ofthe reflectance are used to perform the blood analysis.

It is to be noted that the present invention is not restricted to anyparticular spectroscopic technique but that any type of opticalspectroscopy can be used. This includes (i) infra-red spectroscopy, inparticular infra-red absorption spectroscopy, Fourier transforminfra-red (FTIR) spectroscopy and near infra-red (NIR) diffusereflection spectroscopy, (ii) scattering spectroscopy techniques, inparticularly Raman spectroscopy, stimulated Raman spectroscopy, coherentanti-Stokes Raman spectroscopy (CARS), fluorescence spectroscopy,multi-photon fluorescence spectroscopy and reflectance spectroscopy, and(iii) other spectroscopic techniques such as photo-acousticspectroscopy, polarimetry and pump-probe spectroscopy. Preferredspectroscopic techniques for application to the present invention are IRabsorbance spectroscopy and fluorescence spectroscopy.

In accordance with a further preferred embodiment of the invention thecatheter head has means for analysis of the return radiation beingadapted to perform a spectroscopic analysis, such as Raman spectroscopicanalysis, infra-red absorption spectroscopic analysis, scatteringspectroscopic analysis, fluorescence spectroscopic analysis.

In accordance with a further preferred embodiment of the invention theradiation that is directed to the volume of interest is selected tocause molecular vibrational scattering in order to provide the returnradiation. For example, the radiation is laser radiation or infraredradiation.

In accordance with a further preferred embodiment of the invention aremote controllable shutter is arranged in front of a mesh. The mesh hasa size that prevents red and/or white blood cells to enter the detectionvolume.

In accordance with a further preferred embodiment of the invention amirror is used for the radiation and/or the return radiation, the mirrorbeing a spherical mirror or an ellipsoidal mirror.

In accordance with a further preferred embodiment of the invention thecatheter head has a first optical wave guide for directing of the laserradiation to the blood detection volume and a second optical wave guidefor receiving of the Raman scattered radiation for transmission to themeans for spectroscopic analysis.

In accordance with a further preferred embodiment of the invention thefirst optical wave guide determines an excitation light path and thesecond optical wave guide determines a detection light path, and furthercomprising means for decoupling the excitation light path and thedetection light path.

In accordance with a further preferred embodiment of the invention thecatheter head has means for filtering out of the laser radiation in thedetection light path.

In another aspect the invention concerns a catheter system having acatheter head, the catheter comprising at least one optical wave guidefor coupling of the catheter head to a Raman laser source and to meansfor spectroscopic analysis.

In still another aspect the invention concerns an imaging system forcatheterisation comprising a catheter system and having display meansfor display of a blood property being detected by the spectroscopicanalysis.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention will bedescribed in greater detail by making reference to the drawings, inwhich

FIG. 1 is a block diagram of a catheter system of the invention,

FIG. 2 is illustrative of the catheter head of the catheter system ofFIG. 1 in a blood vessel,

FIG. 3 to 13 show various embodiments of catheter heads,

FIG. 14 shows a block diagram of an imaging system for catheterisation.

DETAILED DESCRIPTION

FIG. 1 shows a catheter system 100 having a catheter head 102. Catheterhead 102 comprises optical fibre 104 which extends through catheter 106.Further, catheter head 102 has objective lens 108 for directing ofradiation towards a detection volume and for collecting of Ramanscattered radiation.

Optical fibre 104 is coupled to optical fibre 110. Optical fibre 110conducts laser beam 112 provided by Raman excitation laser 114 throughconnector 116 to optical fibre 104. Laser beam 112 is directed towards adetection volume through objective lens 108. The Raman scatteredradiation is collected by objective lens 108 and coupled into opticalfibre 104.

The Raman scattered radiation travels through optical fibre 104,connector 116, optical fibre 110 to mirror 118, from where the Ramanscattered radiation 120 is provided to Raman spectrum analyser 122.Raman spectrum analyser 122 analyses the spectrum of the received Ramanscattered radiation 120 in order to determine one or more bloodproperties such as the concentrations of glucose, glycohemoglobin,lactate, bilirubin, cholesterol, triglycerides, hemoglobin and bloodgases.

Further, a variety of other catheter inputs 124 can be connected tocatheter head 102 via connector 116 and catheter 106 depending on thepurpose of the catheterisation such as PTCA or others (cf. U.S. Pat. No.5,938,582 or 6,302,866). Usually each application requires its ownspecial catheter while some functionalities can be combined in speciallydesigned catheters.

FIG. 2 shows catheter head 102 of catheter system 100 of FIG. 1 inoperation. Catheter head 102 has been introduced into blood vessel 200by means of catheter 106. A laser beam is directed through optical fibre104 to the confocal detection volume 202 which is defined by objectivelens 108. Raman radiation is scattered back by the blood flowing throughthe confocal detection volume 202 which is collected by objective lens108 and coupled into optical fibre 104.

FIGS. 3 to 13 show various preferred embodiments of catheter heads forusage in a catheter system of the type as shown in FIGS. 1 and 2.

FIG. 3 shows catheter head 300 which is similar to catheter head 102 ofFIGS. 1 and 2. Catheter head 300 has an elongated housing 302 with anopening for receiving objective lens 304. Optical fibre 306 (cf. opticalfibre 104 of FIGS. 1 and 2) serves to conduct laser radiation throughcatheter 308 which is directed through objective lens 304 towards theconfocal detection volume 310. Raman radiation which is back scatteredfrom detection volume 310 into the direction of objective lens 304 iscoupled back into optical fibre 306 for transmission to the Ramanspectrum analyser (c.f. Raman spectrum analyser 122 of FIG. 1). However,it is to be noted that the elongated form of the housing is notessential for the present invention.

In the following preferred embodiments of FIGS. 4 to 12, alike elementswill be designated by the same reference numerals as in FIG. 3.

In the embodiment of FIG. 4, a cavity 312 is formed in housing 302.Through an opening which is formed in housing 302 blood can flow intocavity 312. Confocal detection volume 310 is located inside cavity 312.In the example considered here objective lens 304 is arranged on one ofthe side walls of cavity 312. This way the surface of objective lens 304is protected against contamination from the vessel walls as the catheterhead 300 moves through the vessel.

In the embodiment of FIG. 5 mesh 314 is disposed at the opening ofcavity 312 towards the blood vessel. Mesh 314 filters out red and/orwhite blood cells. When mesh 314 is closed only blood plasma enterscavity 312. This reduces the absorption and scattering of the Ramanlaser light and Raman signal by the red and/or white blood cells.

In order to filter out the red and/or the white blood cells a mesh sizeof below 5 microns is selected.

In addition shutter 316 can be placed in front of the mesh 314. Thisprevents the mesh from being contaminated while the catheter head 300 ismoved through the blood vessels. Shutter 316 is remote controlled and isonly opened before a blood measurement to enable blood flow throughcavity 312.

In the embodiment of FIG. 6 a blood channel 318 is formed in housing 302which enables a flow of blood through catheter head 300 passing byconfocal detection volume 310. This has the advantage that the surfaceof objective lens 304 can be protected against contamination and thatthe flow of blood through the detection volume 310 is enhanced at thesame time.

Channel 318 can be realised by means of a tube running through catheterhead 300. Alternatively, it is also possible to use a groove along theside of the catheter head 300. In the preferred embodiment of FIG. 7 theblood flow is further enhanced by using the Pitot tube effect. For thispurpose channel 318 has one opening at the front of housing 302 and oneopening at the side of housing 302. This way an extra pressuredifference is created when the blood flows along housing 302 whichenhances the blood flow through channel 318 and through detection volume310.

In the preferred embodiment of FIG. 8 a spherical mirror 320 is locatedopposite to objective lens 304. Detection volume 310 is located withincavity 312 between objective lens 304 and spherical mirror 320. Thelaser light which is directed towards detection volume 310 throughobjective lens 304 is reflected back into detection volume 310 byspherical mirror 320. As a consequence Raman scattering takes placetwice, once for the original laser beam and once for the reflected laserbeam. Further the Raman scattered radiation is also reflected byspherical mirror 320 and collected by objective lens 304; as a resultthe sensitivity and the Raman signal to noise ratio are substantiallyincreased.

In the embodiment of FIG. 9 an ellipsoidal mirror 322 is disposed withinhousing 302. Distal end 324 of optical fibre 306 is located at one ofthe focal points of ellipsoidal mirror 322. Detection volume 310 islocated at the other focal point of ellipsoidal mirror 322. Blood flowsto the detection volume 310 through cavity 312 which extends intoellipsoidal mirror 322 and prevents a complete flooding of ellipsoidalmirror 322 with blood.

In the embodiment of FIG. 10 mirror 326 is completely filled with bloodthrough opening 328 in housing 302. Mirror 326 can be an ellipsoid or aspherical mirror. In this instance, detection volume 310 is located atthe orifice of the optical fibre 306.

In the preferred embodiment of FIG. 11 separate optical fibers 306 and330 are used for guiding of laser radiation to detection volume 310 andfor transmitting of the Raman scattered radiation back to the Ramanspectrum analyser (cf. Raman spectrum analyser 122 of FIG. 1),respectively. Raman scattered radiation is collected by objective lens332 which is perpendicular to objective lens 304 for decoupling.Alternatively another angle can be used. This way the amount of laserlight which is coupled into optical fibre 330 is reduced. For furtherreduction of the laser light in optical fibre 330 a filter 332 can belocated between optical fibre 330 and objective lens 332 to suppress theexcitation wavelength. This has the advantage that the Raman scatteredradiation is not overlaid by fluorescence.

When only a single optical fibre is used both for the Raman excitationlaser beam and the Raman scattered radiation return beam the problem isthat the excitation laser beam can create some amount of fluorescence inthe optical fibre. This fluorescence has a negative influence on thesignal to noise ratio of the Raman signal. By decoupling the Ramanexcitation laser beam and the return beam this problem is solved as thevery low intensity Raman return beam does not create fluorescence in thereturn optical fibre 330. As a consequence the signal to noise ration isimproved in comparison to the embodiments using only a single opticalfibre.

FIG. 12 shows an alternative way of decoupling the Raman excitationlaser beam and the return beam. Dichroic mirror 440 is positioned in thelight path of the Raman excitation laser beam. At the wavelength of theRaman excitation laser beam, e.g. 785 nm, dichroic mirror 440 istransparent.

The Raman scattered radiation is reflected from dichroic mirror 440 asdichroic mirror 440 is reflective at the wavelength of the Ramanscattered radiation, e.g. 800 to 1000 nm. Dichroic mirror 440 reflectsthe Raman scattered radiation onto mirror 442, which can also bedichroic. From mirror 440 the Raman scattered radiation is coupled intooptical fibre 330. No or only a limited fraction of the Raman excitationlaser beam is coupled into optical fibre 330 as at least dichroic mirror440 is transparent to the Raman excitation laser beam.

In the embodiment of FIG. 13 blood channel 336 which is arranged inhousing 302 has half-round shape 338 around the detection volume 310.The orifice of the optical fibre 306 is located at the centre of theflat side of half-round shape 338. The half-round shape is covered witha reflective coating and acts as a spherical mirror. The diameter of thetubular portion of channel 336 is as small as possible to limitabsorption of blood. Again using a Pitot type of tube form enhances theblood flow.

FIG. 14 shows an imaging system 400 having an X-ray component 402 foracquisition of image data. X-ray component 402 is coupled to imagingcomponent 404 for processing of the image data. The output of imagingcomponent 404 is coupled to display unit 406. Such imaging system areknown from the prior art for monitoring of catheterisation. In additionto prior art imaging systems catheter system 100 (cf. FIG. 1) is coupledto imaging component 404. Catheter system 100 provides blood analysisdata to imaging component 404. The blood analysis data is integratedinto the picture which is generated by imaging component 404 anddisplayed on display 406. This way an operator is provided with bothimaging data as well as chemical analysis data for improved monitoringof the state of the patient's body.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

REFERENCE NUMERALS

-   100 catheter system-   102 catheter head-   104 optical fibre-   106 catheter-   108 objective lens-   110 optical fibre-   112 laser beam-   114 raman excitation laser-   116 connector-   118 mirror-   120 raman scattered radiation-   122 raman spectrum analyser-   124 catheter inputs-   200 blood vessel-   202 detector volume-   300 catheter head-   302 housing-   304 objective lens-   306 optical fibre-   308 catheter-   310 detection volume-   312 cavity-   314 mesh-   316 shutter-   318 channel-   320 spherical mirror-   322 ellipsoidal mirror-   324 distal end-   326 mirror-   328 opening-   330 optical fibre-   332 objective lens-   334 filter-   336 channel-   338 half-round spherical mirror-   440 dichroic mirror-   442 mirror-   400 imaging system-   402 x-ray component-   404 imaging component-   406 display unit

1. A catheter head comprising: at least one waveguide for directingradiation to a blood detection volume and for directing of returnradiation to a spectroscopic analyzer that determines at least oneproperty of the blood; an objective lens for receiving the returnradiation from the blood detection volume; a housing, the detectionvolume being located inside a cavity of the housing, the housing havinga sidewall with an opening therethrough to enable the blood to flow intothe cavity; a concave mirror at least partially forming an end of thecavity opposite to an end of the waveguide, the concave mirror beingpositioned relative to the at least one waveguide such that radiationemitted from the at least one waveguide: (1) in part is scattered by theblood, which scattered radiation in part is reflected by the blood backto the end of the at least one waveguide and which scattered radiationin part passes through the blood in the cavity to the concave mirror, isreflected by the concave mirror, and passes back through the blood inthe cavity to the end of the waveguide and (2) in part passes throughthe blood in the cavity is reflected by the concave mirror back in tothe blood in the cavity and is scattered by the blood and the scatteredradiation passes to the end of the at least one waveguide.
 2. Thecatheter head of claim 1 wherein the at least one waveguide is a singlewave guide for guiding of the radiation to the blood detection volumeand for receiving the return radiation for transmission to the means forspectroscopic analysis.
 3. The catheter head of claim 1, wherein theobjective lens is in front of the wave guide and is made from a materialthat reflects Raman scattered radiation.
 4. The catheter head of claim1, further comprising a channel formed through the housing and definingat least in part the cavity, wherein the channel has at least twoopenings with a first opening along the sidewall.
 5. The catheter headof claim 1, wherein the concave mirror includes a spherical mirrorconfigured to reflect the Raman scattered radiation back into the blooddetection volume.
 6. The catheter head of claim 1, wherein the concavemirror includes an ellipsoidal mirror, the blood detection volumecomprising one of the focal points of the ellipsoidal mirror and adistal end of the wave guide being located at the other focal point ofthe ellipsoidal mirror, the mirror forming the cavity.
 7. The catheterhead of claim 1, further comprising a dichroic mirror that allows theradiation to pass therethrough and reflects the return radiation.
 8. Thecatheter head of claim 1, further including: a shutter which iscontrolled remote from the catheter head to selectively permit andprevent access to the blood detection volume.
 9. The catheter head ofclaim 1 further including: a mesh across the opening in the side wall,the mesh having openings sized to prevent at least one of red and whiteblood cells from entering into the cavity.
 10. A catheter headcomprising: at least one waveguide for directing of radiation to abodily fluid detection volume and for directing of return radiation to aspectroscopic analyzer that determines at least one property of thebodily fluid; an objective lens for receiving of return radiation fromthe detection volume; and a housing, the detection volume being locatedalong a channel through the housing, the channel having at least twoopenings and defining a non-linear path through the housing, thedetection volume being defined in the non-linear channel.
 11. Thecatheter head of claim 10, wherein at least a portion of the channel iscoated to provide the reflective surface.
 12. The catheter head of claim10, wherein an end of the cavity opposite to an end of the waveguide hasa reflective surface thereon.
 13. A method of in vivo analysis of blood,the method comprising: positioning a catheter head in blood in vivo;capturing a plasma portion of the blood in a fluid detection volume of ahousing of the catheter head, including preventing at least one of redand white blood cells from entering the fluid detection volume;generating a Raman excitation radiation; guiding the Raman excitationradiation through the catheter head into the fluid detection volume toproduce scattered radiation; reflecting the scattered radiation using adichroic mirror; and guiding the scattered radiation through thecatheter head to a spectroscopic analyzer to determine at least oneproperty of the plasma portion of the blood.
 14. The method of claim 13,further comprising providing selective access to the fluid detectionvolume using a shutter that is remotely controlled.
 15. The method ofclaim 13, wherein the guiding of the Raman excitation radiation and theguiding of the scattered radiation is performed by separate waveguidesin the catheter head.
 16. A method of in vivo analysis of a bodilyfluid, the method comprising: positioning in vivo a catheter head inproximity to the bodily fluid; drawing a portion of the bodily fluidthrough a housing along a non-linear path; generating a Raman excitationradiation; guiding the Raman excitation radiation through the catheterhead into a fluid detection volume defined in the non-linear path toproduce scattered radiation; and guiding the scattered radiation throughthe catheter head to a spectroscopic analyzer to determine at least oneproperty of the bodily fluid.
 17. The method of claim 16, whereindrawing the portion of the bodily fluid along the non-linear pathincludes using a pitot tube effect, wherein the housing has at least twoopenings for flow of the portion of the bodily fluid.
 18. The method ofclaim 16, further comprising reflecting at least a portion of thescattered radiation into a waveguide that is guiding the scatteredradiation through the catheter head.
 19. The method of claim 16, furthercomprising preventing at least one of red and white blood cells fromentering the fluid detection volume using a mesh.